Shock acceleration of relativistic particles in galaxy-galaxy collisions

TL;DR

This paper models shock acceleration of particles in galaxy collisions, explaining excess radio emission in galaxy bridges without star formation, and predicts associated gamma-ray emission levels.

Contribution

It introduces a simple shock acceleration model for relativistic particles in galaxy-galaxy collisions, linking gas dynamics to nonthermal radio emission independently of star formation.

Findings

Radio emission from galaxy bridges can be explained by shock-accelerated particles.

The model's spectral index matches observed values.

Gamma-ray emission is predicted but likely undetectable with current instruments.

Abstract

All galaxies without a radio-loud AGN follow a tight correlation between their global FIR and radio synchrotron luminosities, which is believed to be ultimately the result of the formation of massive stars. Two face-on colliding pairs of galaxies, UGC12914/5 and UGC 813/6 deviate from this correlation and show an excess of radio emission which in both cases originates to a large extent in a gas bridge connecting the two galactic disks. The radio synchrotron emission expected from the bridge region is calculated, assuming that the kinetic energy liberated in the predominantly gas dynamic interaction of the respective interstellar media (ISM) has produced shock waves that efficiently accelerate nuclei and electrons to relativistic energies. A simple model for the acceleration of relativistic particles in these shocks is presented together with a calculation of the resulting radio emission, its spectral index and the expected high-energy gamma-ray emission. This process is not related to star formation. It is found that the nonthermal energy produced in the collision is large enough to explain the radio emission from the bridge between the two galaxies. The calculated spectral index at the present time also agrees with the observed value. The expected gamma-ray emission, on the other hand, is too low by a factor of several to be detectable even with foreseeable instruments like CTA.